The tissue engineering field is well developed and a variety of tissue equivalents have been developed. Not only do these equivalents vary in the material that is used for a scaffold, but they also vary in the type and source of cells that are capable of regenerating tissue. Skin equivalents are one area of tissue engineering that has witnessed significant research and commercial development.
Modern skin equivalents may be made using a variety of materials and methods. For example, a cultured skin may be prepared by culturing human fibroblasts in collagen gel, followed by inoculating and culturing human keratinocytes on the gel when the gel is shrunk (U.S. Pat. No. 4,485,096). Another skin equivalent is prepared by inoculating and culturing human fibroblast on nylon mesh, followed by inoculating and culturing human keratinocyte thereon when pores of the mesh are filled up with secreted materials from fibroblasts (Slivka, S. R., L. Landeen, Zirriber, M., G. K. Naughton and R. L. Bartel, J. Invest. Dermatol., 96: 544A, 1991). Alternatively, a skin may be prepared by inoculating and culturing human fibroblasts in a collagen sponge, followed by laminating collagen gel or film inoculating and culturing human keratinocyte thereon (J. Jpn. P. R. S., 10, 165-180 (1990) and Japanese Examined Patent Publication No. 47043/1995).
The advanced nature of skin equivalents is also shown by the fact that a number of skin equivalents are commercially available. Commercial skin equivalents include EpiCel™ (which lacks a dermal component and uses the patient's own cultured keratinocytes) Integra™ (which uses a collagen-glycosaminoglycan (GAG) matrix to provide an acellular dermal component and uses a thin epidermal autograft), AlloDerm™, (which uses a dermal matrix and a thin epidermal autograft), DermaGraft™ (which uses a polyglycolic acid/polylactic acid (PGA/PLA) matrix and allogeneic human fibroblasts for the dermis), Hyaff/LaserSkin™ (which uses hyaluran and fibroblasts for the dermis, and hyaluran and the patient's own keratinocytes for the epidermis), and PolyActive™ (which uses polyethylene oxide/polybutylthatate (PEO/PBT) and the patient's own fibroblasts for the dermis, and the patient's cultured keratinocytes for the epidermis).
Other commercially available skin equivalents include ApliGraft™, which uses collagen gel and allogeneic fibroblasts for the dermis, and cultured allogeneic keratinocytes for the epidermis, Comp Cult Skin™ or OrCel™, which uses collagen and allogeneic fibroblasts for the dermis, and cultured allogeneic keratinocytes for the epidermis, and TransCyte™, which uses allogeneic fibroblasts for the dermis and a synthetic material, BioBrane™, for the epidermis.
Skin equivalents have been used to treat a variety of skin defects. Successful regeneration of skin has been observed for injuries such as burns, as well as for skin disorders that result from a disease such as diabetic ulcers.
However, the use of cellular scaffolds (i.e. skin equivalents) is not limited to therapeutic tissue regeneration. Cellular scaffolds also find use as cell-based assays and tissue culture systems.
The use of fibroblasts also presents a challenge to the production of therapeutic tissue models. Although fibroblasts provide growth factors and other cell-to-cell contacts that facilitate cell division, their proliferation may outpace epidermal cell division resulting in a culture that is overgrown with fibroblasts. This is clearly undesirable as therapies aimed at the regeneration of epidermal tissues must be carried out using carriers rich in epidermal cells. One means of preventing the overgrowth of fibroblast involves plating the epidermal cells with irradiated 3T3 (mouse) cells. Rheinwald and Green; Cell, 6, 331-334, November 1975). However this technique requires the presence of dermal components which is undesirable in therapeutic applications. Another approach for producing skin equivalents that are proportionately high in keratinocyte precursors is to seed the cell scaffold with more keratinocytes than fibroblasts. However, precursor keratinocytes produce collagenase when cultured in the absence of keratinocytes. Thus, seeding collagen scaffolds with precursor keratinocytes results in scaffold degredation unless fibroblasts are present.